International Astrobiology Workshop 2013 (2013)
1010.pdf
Unique Late Archean Atmosphere due to Enhunced Volcanic and Biological Activities
Y. Ueno1,2,3 , S.O. Danielache4 and Y. Endo1
1
Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro Tokyo 152-8551, Japan
2
Earth-Life Science Institute, Tokyo Institute of Technology, Meguro Tokyo 152-8551, Japan <[email protected]>
3
Precambrian Ecosystems Laboratory, JAMSTEC, Japan.
4
Faculty of Science & Technology, Sophia University, Japan.
Late Arcehan may be characterized by increasing
continental volume and large igneous provinces as well
as by onset of oxygenic photosynthesis [1,2].
Enhanced biological production at that time may not
have readily resulted in oxidizing atmosphere, and
rather caused increasing biological methane in the
atmosphere [3,4], that is supported by anomalously
13
C-depleted organic carbon recorded in the Late
Archean sequence [5]. The large volcanic input into
CH4-bearing very reducing atmosphere may cause
unique atmosphere of habitable planet.
In order to test the scenario, we have developed a
sulfur isotopic model by improving our atmospheric
reaction model [6,7]. The improvements to our model
includes the addition of hydrocarbon chemistry,
chemical formation and deposition of organic sulfur
haze, together with newly determined high-accuracy
ultraviolet absorption cross sections of SO2
isotopologues for reproducing the geological record.
The “Sulfur Mass-Independent Fractionation” (S-MIF)
has been useful to monitor chemistry of the Earth’s
early atmosphere. Sedimentary sulfides exhibit
exceptionally large variation of Δ33S values in the
latest Archean, from 2.7 to 2.5 Ga, compared to older
period. The maximum scatter of S-MIF may indicate
anomalous chemistry of atmosphere or climatic system
of the late Archean Earth, though the primary cause of
the large MIF is still poorly understand.
Our model results suggest that after a volcanic
injection of SO2 into the Archean atmosphere, a
significant fraction of the sulfur is converted into
carbonyl sulfide (OCS) and could be accumulated in
an atmosphere over a timescale of 10 years, if
background atmosphere is reducing enough to yield
hydrocarbon haze and volcanic sulfur input is large
and episodic. Such model could explain the large Δ33S
scatter observed in the Late Archean sedimentary rocks.
Moreover, isotopically fractionated two reservoirs (i.e.
atmosphere and ocean) can be mixed episodically and
thus possible to explain the observed small scale
heterogeneity of S-MIF even within a hand specimen
level. Combined greenhouse effect by the CH4 and
OCS could have resulted in warm Late Archean
climate. Furthermore, subsequent oxidation event of
this highly reducing atmosphere may have been more
significant for cooling than previously thought, thus
could have been the triger of global-scale glaciation at
around the earliest Proterozoic.
References: [1] Sessions et al. (2009) Current Biology
19, R567–R574, [2] Ernst (2007) Episode 30, 108-113.
[3] Hoeler et al. (2001) Nature 412, 324-327. [4]
Pavlov et al. (2001) Geology 29, 1003-1006. [5] Hayes
(1994) Early Life on Earth. pp. 220-236. [6]
Danielache et al. (2008) J Geophys Res 113, D17314.
[7] Ueno et al. (2009) PNAS 106, 14784-14789.